Component alignment is of critical importance in semiconductor and/or MEMS (micro electromechanical systems) based optical system manufacturing. The basic nature of light requires that light generating, transmitting, and modifying components must be positioned accurately with respect to one another, especially in the context of free-space-optical systems, in order to function properly and effectively in electro-optical or all optical systems. Scales characteristic of semiconductor and MEMS can necessitate micron to sub-micron alignment accuracy.
Consider the specific example of coupling a semiconductor diode laser, such as a pump or transmitter laser, to a fiber core of a single mode fiber. Only the power that is coupled into the fiber core is usable. The coupling efficiency is highly dependent on accurate alignment between the laser output facet and the core; inaccurate alignment can result in partial or complete loss of signal transmission through the optical system. 
Other more general examples include optical amplification, receiving and/or processing systems. Some alignment is typically required between an optical signal source, such as the fiber endface, and a detector. In more complex systems, including tunable filters, for example, alignment is required not only to preserve signal power but also to yield high quality or high finesse systems through the suppression of undesirable optical modes within and without the systems.
Generally, there are two types of alignment strategies: active and passive. Typically in passive alignment of the optical components, registration or alignment features are fabricated directly on the components or component carriers as well as on the platform to which the components are to be mounted. The components are then mounted and bonded directly to the platform using the alignment features. In active alignment, an optical signal is transmitted through the components and detected. The alignment is performed based on the transmission characteristics to enable the highest possible performance level for the system.
In the context of commercial volume manufacturing, selection between active and passive alignment, or some mix of the two, is determined based on the quality of part needed versus the margins for the part. Lower cost, lower performance devices are typically manufactured with entirely passive alignment strategies, whereas the manufacture of high performance devices typically involves at least some active alignment.